Change Data Labels (XP)

Hi there,

I want to create a bar chart that graphs out the actual numbers in my source data, but I want the data labels to show what percentage of the whole these numbers represent. I know I can manually type in data labels, but this takes time. If I work out the percentages on the worksheet next to my source data, is there a way to tell the chart to go find the percentages and use them as the data labels?
Thanks